Solar water-heating contributes substantially to the ecology and to saving fossil resources. This presumes well-operating, rugged systems that are as passive as possible, i.e., do not require auxiliary energy inputs. Solar water-heaters with directly affixed storage tanks have been recently the most successful on the market. These known solar water-heater systems essentially constitute
thermo-siphon systems, and PA1 systems wherein a collector and a storage device form a single unit. PA1 relatively high absorber intake temperatures exist and therefore high heat losses are incurred, PA1 after the storage unit is completely discharged, hot water will be available only following a relatively time consuming heating stage.
Regarding efficiency, these systems evince the drawback that most of the time they are mixed-storage types wherein the temperature of all the water is raised more or less uniformly in the storage unit by a mixing assembly. However, to be useful as a water-heater, the temperature must always be above a minimum value. If hot water is removed, a corresponding amount of cold water will be added to the hot water. Therefore, the following drawbacks ensue:
Occasionally layer storage is used in combination with thermo-siphon systems to overcome the aforementioned drawbacks. However such systems are not compact and their manufacture and installation are more expensive. Furthermore, they still evince some temperature balance and hence loss of efficiency.
Another drawback of thermo-siphon equipment is their unsightliness because of superposed and visible tanks. This drawback is circumvented in integrated absorber-tank units (the so-called storage collectors). As a rule, in such storage collectors, the water is held in a well insulated storage unit and is bounded at its surface by an absorber wall. In turn this absorber wall is covered with one or more transparent layers (transparent insulation) toward the sun side. The incident solar radiation heats the absorber wall and this wall, in turn, transfers part of this heat to the storage water below and the remainder through radiation, conduction and convection through the transparent thermal insulation to the ambient.
In this regard a problem is encountered in that, on the one hand, the transparent cover must be highly insulated to prevent intense nocturnal cooling but, on the other hand, it must be highly transparent. Presently available transparent heat insulating materials either strike an unsatisfactory tradeoff, or they are comparatively expensive.
A storage collector solving the problems of excessive tank temperatures is described in European patent Publication No. 219,566 dated Oct. 22, 1985 which discloses a variable-volume storage to which cold water is admitted thermostatically, whereby the water temperatures always remain approximately constant. However the addition of cold water results in unnecessarily high heat losses which are reflected in the storage temperatures.
A storage collector free of the above drawbacks is described in German published patent application 3,728,551.3. In this arrangement, the topological design is nearly identical with a conventional storage collector. The essential difference is that a thermal insulating layer is present between the surface-mounted collector and the integral storage below. Accordingly heat transfer can no longer take place directly through the wall into the storage water, but rather the heat transfer is carded out through a small circulating pump feeding the temperature-gradient water in the tank from the lower colder zone into the absorber from where the water, upon reaching a desired temperature, is fed in laminar flow into the top of the tank.
In spite of its compactness, such a storage collector offers excellent thermal insulation for the storage part and good optical properties for the collector. Nevertheless, it incurs two substantial drawbacks: the assumption of stable temperature layering for rapid availability and low heat losses by the collector is impossible on account of the flat design, and the system cannot operate passively.